The present disclosure relates to a fixing device configured to fix an toner image on a sheet and an image forming apparatus including the fixing device.
In an image forming apparatus, an image carrier such as a photosensitive drum is charged and then exposed based on image data to form an electrostatic latent image on the surface, and the electrostatic latent image is developed into a toner image by a development unit. The toner image formed on the surface of the image carrier is transferred to a sheet and then fixed to the sheet by a fixing device.
Some of the fixing devices are configured to heat a belt-like or a roller-type fixing member by a heat source and to heat and press the toner image between the heated fixing member and a pressing member, thereby melting and pressing the toner image to fix on the sheet.
In recent years, there is a tendency to reduce a heat capacity of the fixing member in response to an environmental issue. As a measure to reduce a heat capacity, the fixing member may be made thin. However, if the fixing member is made thin, there arises such a problem that a heat conductivity thereof in a direction (a sheet width direction) intersecting with a sheet conveying direction deteriorates and temperature unevenness is easy to occur in the sheet width direction.
The fixing member is configured such that the entire region in the sheet width direction is heated regardless of sheet size. Therefore, after a small size sheet passes through the fixing member, while a temperature of the sheet passing region lowers because a heat quantity is transmitted to the sheet, a heat quantity in a non-sheet passing region outside of the sheet passing region is not transmitted to the sheet but remains. If a heat conductivity in the sheet width direction is low, since the remaining heat quantity is hardly transmitted in the sheet width direction, unevenness of temperature occurs between the small size sheet passing region and the non-sheet passing region. Then, if a large size sheet passes through after passing the small size sheet, image deterioration called as a hot offset phenomenon or unevenness of gloss caused by the unevenness of temperature are easy to occur. Still further, because the heat quantity remains locally, there is a case when the fixing member or a member arranged near the fixing member is deteriorated by the heat.
To cope with this problem, the fixing device may be controlled such that the fixing operation waits until the unevenness of temperature is eliminated by lowering a sheet conveying speed or temporarily stopping the conveyance of the sheet. However, if the waiting time is long, there arises a problem that a printing productivity of small size sheet deteriorates.
Then, there is a fixing member configured to have a two-layer tubular structure in which a graphite layer having an anisotropic heat conductivity is provided between the two layer tubes to increase the heat conductivity in an axial direction.
However, this fixing member has a problem that it is costly because the configuration of the multi-layer tubular structure is complicated and its molding steps increases. There is also another problem that a power consumption increases and a power saving performance deteriorates because a heat capacity increases by adding the graphite layer.